Electrolytic cells are commonly used to make chlorine and an alkali metal hydroxide solution by electrolysis of an alkali metal chloride solution. Such cells are known as chlor-alkali cells. Reference is made in this specification to chlor-alkali cells and processes as typifying electrolytic cells and processes in general.
There are three broad types of chlor-alkali cell, "mercury", "diaphragm" and the more recently developed "membrane" cells.
In membrane cells the anodes and cathodes are separated by cation-active permselective membranes; these are membranes which are selectively permeable so as to allow the passage of only positively charged ions and not the passage of bulk electrolyte. Cation-active perm-selective membranes which are suitable for this use in chlorine cells include, for example, those made of synthetic organic copolymeric material containing cation-exchange groups, for example, sulphonate, carboxylate and phosphonate. Perm-selective membranes are non-porous.
On the other hand, diaphragm cells, in which the anodes and cathodes are separated by porous diaphragms, permit the passage of both positive and negative ions and of electrolyte.
In operating a diaphragm cell for the electrolysis of alkali metal chloride solutions to give chlorine and alkali metal hydroxides, it is essential that flow of the solutions through the tortuous microporous diaphragm be unimpeded by gas voids in the porous network.
Diaphragms prepared from asbestos fibres have generally been used but these suffer from the disadvantage that
(1) the lifetime of the fibrous asbestos network in the chlor-alkali cells is limited; PA1 (2) the handling of asbestos fibres is often environmentally undesirable; and PA1 (3) the thickness of the fibrous matte limits the extent to which the interelectrode gap can be reduced. PA1 (1) they generally affect adversely the processing of the fluoropolymers into a fibrous or sheet form; PA1 (2) they are readily leached out by flow through the diaphragm; and PA1 (3) initial wetting out of the tortuous microporous network is difficult to achieve satisfactorily.
Alternatively diaphragms comprising fluoropolymer materials in sheet or in fibrous form which are inert to the cell liquors have been proposed. However, these diaphragms suffer from the problem that they are hydrophobic and difficult to wet with alkali metal chloride and hydroxide solutions and in consequence tend to have gas-filled voids in the porous network of the diaphragm. This can lead to diaphragm blockage, high voltages and mixing of the product gases, hydrogen and chlorine.
Several methods have been proposed for rendering such diaphragms hydrophilic. For example UK Patent 1,081,046 and Belgian Patent 794,889 to ICI Ltd describe processes for microporous sheet diaphragms in which a hydrophilic particulate inorganic additive such as titanium dioxide is added to confer hydrophilicity on the diaphragm matrix. Other additives such as surface active agents have also been proposed for this purpose.
These additives suffer from the disadvantage that
It is an objective of the present invention to provide hydrophilic fluorocarbon membranes for use in chlor-alkali electrolysis cells.
We have now devised a process of rendering hydrophobic fluorocarbon polymers hydrophilic by means of graft copolymerization of certain monomers to the hydrophobic fluorocarbon polymer substrate.